The invention relates to a microwave frequency device comprising at least a transition between a transmission line integrated on a substrate, disposed in a first microwave frequency cavity, and a waveguide formed by a second microwave frequency cavity, this transition comprising an open end of the integrated line which end forms a probe inserted into the cavity of the waveguide, at a distance from a short-circuit which closes off the end of the waveguide, this transition further comprising an impedance adapting system.
The invention finds its application in microwave frequency devices which comprise integrated circuits and waveguides, which are to be connected to each other. The invention is useful, for example, in the fields of television aerials and automotive radar.
A transition between a waveguide and a microstrip line is already known from the publication in "1988 IEEE MTF-S Digest, P.4, pp. 473-474", entitled "WAVEGUIDE-TO-MICROSTRIP TRANSITIONS FOR MILLIMETER-WAVE APPLICATIONS" by Yi-Chi SHIH, Thuy-Nhung TON, and Long Q. BUI, of Hughes Aircraft Company, Microwave Products Division, TORRANCE, Calif., U.S.A.
This publication describes a transition between a microwave frequency microstrip line inserted into a first microwave frequency cavity, and a waveguide formed by a second microwave frequency cavity. This transition has an open end of the integrated line which extends into the waveguide perpendicularly to the waveguide propagation axis, through an opening made in a wall of the waveguide. In this manner the propagation planes of the electric field E of the probe and of the waveguide coincide. This transition further includes an impedance adapting system applied to the integrated line which system consists of a narrowing over a specific length of the microstrip at the surface of the substrate. This length is provided to form a quarter wave adapter to bring the input impedance of the probe to 50 .OMEGA.. The end of the waveguide forming the short-circuit is situated at a distance L from the microstrip conductor and the end forming the probe of the latter extends into the waveguide to a depth D. By minutely adjusting these dimensions, the known device can be a broadband device in the K band (18 to 26 GHz).
To date planar integrated circuits operating with microwave frequencies between 40 GHz and 100 GHz have been used more and more in the field of telecommunications. These integrated circuits generally include planar transmission lines, for example, microstrip lines and are interconnected or connected to aerial elements by means of waveguides.
These planar integrated circuits operating with such high frequencies require appropriate housings which are capable of retaining their performance. They additionally require devices capable of realizing a transition between their input/output stubs and the connecting waveguides.
As regards the housings, they are to present eminent microwave qualities, which qualifies are specific of the operating frequency of the circuits. The accent is to be laid particularly on the tolerances of the ground stubs and on the tolerances of the microwave frequency links between the input/output stubs of the integrated circuits and the external elements, links which are to be realized by, for example, gold conducting wires which are very short and very fine and are bonded on the various stubs, for example, by means of thermocompression. The accent is also to be laid on the mechanical resistance and the imperviousness of the housings which are to safeguard the integrated circuits from dust and from corrosion, which hazards are susceptible of deteriorating the electrical qualities of the housings; in effect, numerous microwave frequency circuits used in telecommunications are positioned on aerial mounts or on vehicles and are thus subject to bad weather conditions.
As regards the devices that realize a transition from a waveguide to a transmission line, they are to be compatible with both the standard waveguides and the microwave frequency inputs/outputs of the integrated circuits. Furthermore, these devices are to present all the mechanical qualities and electrical qualities defined hereinbefore for the housings. More particularly, these devices are to be sealed and not to cause sealing discontinuities occurring between the waveguides and the integrated circuits. The electrical connections between this type of transition and a given integrated circuit are to meet the requirements defined hereinbefore relating to the tolerances of the microwave frequency stubs and ground stubs.
In addition, these transition devices are to present a good adaptation, in a wide frequency band, at frequencies as high as 40 Ghz to 100 GHz.
The device known from said document does not yield a waveguide-to-transmission line transition:
that permits a sealed link between a waveguide and an integrated circuit, PA1 that makes it possible to manufacture microwave frequency links to an integrated circuit that have the required tolerances, PA1 that presents an adaptation to the considered microwave frequencies which is easy to realize from a point of view of manufacturing. PA1 the adapting means which are applied to the cavity of the waveguide and also to the cavity of the line permit using a substrate that has about twice the transversal dimension of that of the state of the art, which results in that this substrate can be hard; PA1 the substrate having a high dielectric constant could be a hard material which permits obtaining bonds on the conductor of the line by means of thermocompression and thus obtaining good microwave frequency contacts; PA1 the substrate which is hard and wider than the known substrate is suitable for extending crosswise over the whole cross section of the waveguide to close off the waveguide and thus seal the cavity of the line, this closing off being all the more simple if the cavity of the waveguide has small dimensions in this area. PA1 the sealed cavity of the line can accommodate an integrated circuit; PA1 this integrated circuit will have a high-quality microwave frequency link to the line because of the hard substrate; PA1 the adapting system of this transition device is better than the prior-art adapter; PA1 the operating frequency band of this transition device can also be widened considerably.
In effect, with the prior-art waveguide-to-transmission line transition device it is not possible to provide that the sealing required for the microstrip line is not interrupted. This microstrip line is realized on a substrate by way of an integrated circuit technique. The cavity embedding the line is thus to be sealed relative to the waveguide for reasons given hereinbefore. The plane substrate supporting the end that forms a probe extended into the waveguide does not close off the cavity of the waveguide because the transversal dimension of the substrate is smaller than the width "a" of the cross section of the waveguide.
Then, the electrical match is hard to realize. To realize the transition, the line is to extend into the waveguide cavity of the over length D, which is smaller than dimension "b" of the waveguide. The end of the line thus forms an open circuit that radiates. Therefore, a metallic plane, forming a short-circuit for the waveguide and closing off this waveguide perpendicularly to the direction of propagation so as to ensure a maximum propagation of the radiated power in this transition, is judiciously to be disposed at a distance L=.lambda./4 from said line. The radiation can thus be controlled by the length L of the short-circuit which is fixed. This transition makes it necessary to include an impedance transformer that consists of a narrowing of the microstrip conductor near to the probe. This type of technology is hard to implement in industry when the designer with respect to microwave frequency devices is confronted with the problem of realizing consumer devices, as is the case in the field of television or automotive vehicles. Thus it is necessary that the performance obtained is not vulnerable to the manufacturing tolerances; in the case of this narrowing of the conductor, it is.
Furthermore, the substrate used for realizing the prior-art device is made of a supple material (Duroid) which has several particular features. In the prior-art device this supple substrate is used for two reasons: the first reason is that the transversal dimensions of the substrate are to be very small for reasons of adaptation, and that only a supple substrate can support such small dimensions; the second reason is that the supple substrates have a low dielectric constant of the order of 2, whereas the hard substrates, such as alumina, have a higher dielectric constant of the order of 8 to 10, which is remote from the dielectric constant of air (1). It happens that this supple substrate is a drawback for realizing microwave frequency electrical connections by very fine gold wires because of its suppleness, the technology of fixing wires by thermocompression cannot be used. Realising interconnections between a supple substrate and a chip or an integrated circuit of a hard substrate is a problem that the expert has so far not been able to solve. This type of interconnection is thus to be avoided if the designer of microwave frequency devices wishes to count on a good manufacturing output.
The dimensions used in the state of the art are to be considered well. With reference to FIG. 1A of said document, the large dimension "a" of the waveguide is 3.8 min. The substrate inserted into the waveguide is much narrower: its width is about half of "a", that is 1.9 min. The distance between the two waveguides in the double-transition structure also described in said publication is 18 min. In this prior-art arrangement the dimensions of the substrate are thus ultimately 1.9 mm.times.18 mm. These dimensions make the substrate very fragile. Therefore, in the prior-art arrangement the substrate cannot be manufactured of a material other than a supple material.